Location Sensor

As their name suggests, the Location Sensor detects the location of something, which means that they are referenced to a fixed point or location or from that point.Such sensors provide "positional" feedback.

One method of determining a location is to use "distance" or "rotation" (angular motion), which can be the distance between two points, such as the distance traveled or moving away from a fixed point.For example, the rotation of a robot wheel to determine the distance traveled on the ground.In both cases, Position Sensors can detect the movement of an object in a straight line using Linear Sensors or angular motion using Rotation Sensors.

Potentiometer

The most widely used of all "Position Sensors" is the ponciometer,as it is an inexpensive and easy-to-use position sensor.There is a scraper contact that can be angular (rotary) or linear (slider type) in its movement, causing the resistance value between the wiper/slider and the two end connections to change and connected to a mechanical shaft by giving an electrical signal. There is a proportional relationship between the actual wiper position on the resistance track and the resistance value. In addition, resistance is proportional to the position.

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Potentiometer

Potentiometers come in a wide range of designs and sizes, such as the widely available round-rotation type or longer and straight linear slider types.When used as a position sensor, the moving object is connected directly to the rotating shaft or slider of the ponciometer.

A DC reference voltage is applied to the two external fixed connections that make up the resistance element.The output voltage signal is taken from the wiper terminal of the sliding ignot, as shown below.

This configuration produces a potential or voltage divider type circuit output that is proportional to the shaft position.For example, if you apply a voltage of 10v to the resistant element of the ponsiometer, the maximum output voltage will be equal to the 10 volt feed voltage, with the minimum output voltage equal to 0 volts.Then the poninciometer wiper changes the output signal from 0 to 10 volts, and 5 volts indicate that the wiper or slider is halfway or in the center position.

Ponsiometer Structure

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The output signal (Vout) from the ponciometer is taken from the center wiper connection when moving along the resistance path and is proportional to the angular position of the shaft.

Example of a Simple Location Detection Circuit

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Location Sensor

Resistant ponciometer position sensors have many advantages: low cost, low-tech, easy to use, etc. As a position sensor, it also has many drawbacks: wear from moving parts, low accuracy, low repeatability and limited frequency response.

But there is one main drawback to using the posiometer as a position sensor.The range of motion of the wiper or slider (and therefore the resulting output signal) is limited by the physical size of the pontiometer used.

For example, a single-turn rotational ponciometer typically has a maximum of 240 with only 0oit has a constant mechanical rotation from 330o. However, 3600o (10 x 360o)is available on multi-round ponciometers until mechanical rotation.

Most types of ponciometers use carbon film for their resistant traces, but these species are electrically noisy (sizzle on a radio sound control) and also have a short mechanical lifescrew.

Wire-winded containers in the form of flat wire or winding coil-resistant wire, also known as reostas, can also be used, but wire-bandaged models suffer from resolution problems (LOG) output, causing errors in the output signal, as the wipers jump from one wire segment to another by producing a logarithmic.They also suffer from electrical noise.

Conductive plastic resistant element type polymer film or sermet type posensiometers are available for high precision low noise applications.These models have a smooth, low-friction electrically linear (LIN) resistant pathway that gives them low noise, long life and excellent resolution, and are available as both multi-turn and single-turn devices.Typical applications of such high-precision position sensors are computer game controllers, steering wheel, industrial and robotic applications.

Inductive Position Sensors

Linear Variable Differential Transformer

It is a "Linear Variable Differential Transformer", or LVDTfor short, which is a positional sensor that is not affected by mechanical wear problems.This is an inductive type position sensor that operates on the same principle as the AC transformer used to measure movement.It is a very sensitive device for measuring linear displacement, and its output is proportional to the position of its moving core.

Basically, it consists of three coils, one wrapped in a hollow one that forms the primary coil, and the other two coils, which are serially connected together, but form identical secondary onesthat come out of the phase on both sides of the primary coil by 180.

Within the tubular body of the LVDT, it is called a moving iron ferromagnetic core fixture, which is connected to the measured object, sliding or moving up and down.

A small AC reference voltage called a "warning signal" (2 – 20V rms, 2 – 20kHz) is applied to the primary winding, which induces an EMF signal to two adjacent secondary windings.

If the iron magnetic core fixture is exactly in the center of the tube and the windings are in an "empty position", the two induced emks in the two secondary windings cancel each other out because they are 180 o-phases , so the resulting output voltage is zero.When the core is slightly shifted from one side or another from this zero position, the induced voltage in one of the secondary will be larger than the other secondary and an output will be produced.

The polarity of the output signal depends on the direction and displacement of the moving core.The larger the movement of the iron core from its central empty position, the greater the resulting output signal.The result is a differential voltage output that changes linearly with the core position.Therefore, the output signal from this type of position sensor has both an amplitude, which is a linear function of core displacement, and a polarity indicating the direction of movement.

The phase of the output signal is comparable to the primary coil excitation phase, which allows suitable electronic circuits such as the AD592 LVDT Sensor Amplifier to know which half of the magnetic core is in the coil and therefore the direction of movement.

Linear Variable Differential Transformer

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When the fixture is moved from one end to the other from the middle position, the output voltages return from maximum to zero and again to maximum, but in the process the phase angle changes by 180 degrees.This allows the LVDT to produce an AC output signal, the magnitude of which represents the amount of movement from the central position and the phase angle represents the direction of movement of the core.

A typical application of a linear variable differential transformer (LVDT) sensor will be as a pressure transducer if the measured pressure pushes against a diaphragm to produce a force.The force is then converted into a voltage signal that can be read by the sensor.

The advantages of a linear variable differential transformer or LVDT compared to a resistant ponciometer are that its linearity, that is, the voltage output to displacement, is excellent, very good accuracy, good resolution, high precision, as well as frictionless operation.

Inductive Proximity Sensors

Another widely used type of inductive position sensor is the Inductive Proximity Sensor,also called the Vortex current sensor.Although they do not actually measure displacement or angular rotation, they are mainly used to detect the presence of an object in front of them or nearby, so they are called "proximity sensor".

Proximity sensors are contactless position sensors that use a magnetic field for detection, and the simplest magnetic sensor is the reed switch.In an inductive sensor, a coil is wrapped around an iron core within an electromagnetic field to create an inductive loop.

When a ferrommannetic material is placed in the area of the vortex current created around the inductive sensor, such as a ferromagetic metal plate or metal screw, the inductive of the coil changes significantly.The proximity sensors detection circuit detects this change, which produces an output voltage.Therefore, inductive approach sensors operate under the electrical principle of Faraday's Inductive Law .

Inductive Proximity Sensors

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The inductive proximity sensor has four main components; The oscillator creates an electromagnetic field, the coil creates a magnetic field, this is a purpose of the sensing circuit and is used to detect any changes in the field when it enters, no (normally on) and NC as outputthey have (normally off) pins.

Inductive proximity sensors allow the detection of metallic objects in front of the sensor head without any physical contact with the object itself.This makes them ideal for use in dirty or wet environments.The "detection" range of proximity sensors is very small, typically from 0.1 mm to 12 mm.

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Proximity Sensor

In addition to industrial applications, inductive approach sensors are widely used to control traffic flow by changing traffic lights at intersections.Rectangular inductive wire rings are embedded on the asphalt road surface.

When a car or other road vehicle passes over this inductive loop, the metallic body of the vehicle changes the inducing of the loops and activates the sensor, alerting the traffic lights controller that it is a waiting vehicle.

One main drawback of such position sensors is that they are "Versatile", that is, they will detect a metallic object above, under or next to it.In addition, Capacitive Proximity Sensors and Ultrasonic Proximity Sensors are available but do not detect non-metal objects.Other commonly available magnetic position sensors include: reed switches, Hall Effect Sensors, and variable relux sensors.

Rotary Encoders

Rotary Encoders areanother type of position sensor that resembles the previously mentioned potentiometers but are non-contact optical devices used to convert the angular position of a rotating shaft into analog or digital data code.In other words, they convert mechanical motion into an electrical signal (preferably digital).

All optical encoders operate on the same basic principle.Light from an LED or infra-red light source is passed through a rotating high-resolution encoded disk that contains the necessary code models, such as binary, gray code, or BCD.Photo detectors scan the disc as it rotates, and an electronic circuit converts information into a digital form as a stream of binary output pulses fed into meters or controllers that determine the actual angular position of the shaft.

There are two basic types of rotary optical encoders, Incremental Encoders and Absolute Location Encoders.

Incremental Encoder

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Encoder Disk

Incremental Encoders, also known as quad encoders or relative rotary encoders, arethe simplest of the two position sensors.Its output is a series of square wave pulses produced by a photocell arrangement as a coded disc that moves or rotates next to the light source, with evenly spaced transparent and dark lines called segments on its surface.The encoder produces a stream of square wave pulses that indicates the angular position of the spindle when counted.

Incremental encoders have two separate outputs called "quad outputs".These two outputs are 90 out of phases from each other by determining the direction of rotation of the shaft from the output sequence.

The number of transparent and dark sections or slots on the disk determines the resolution of the device, and increasing the number of lines in the pattern increases the resolution per degree of rotation.Typical encoded disks have a resolution of 256 pulses or 8 bits per turn.

The simplest incremental encoder is called a tachometer.It has a single frame wave output and is usually used only in one-way applications where basic location or speed information is required.The encoder "Quadruple" or "Sinus wave" is more common and has two output square waves, often called channel A and channel B.This device uses two photo detectors with a slightly 90 o offset from each other, thereby producing two separate sinus and cosine output signals.

Simple Incremental Encoder

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Using the tangent arc mathematical function, the angle of the shaft in radians can be calculated. In general, the optical disc used in rotary-position encoders is circular, after which the resolution of the output will be given as follows: ε = 360/n, where n is equal to the number of segments on the encoded disk.

Then, for example, the number of segments required to give the incremental encoder a resolution of 1o will be: 1o = 360/n, hence n = 360 windows(segments), and so on. In addition, the rotation direction is determined by paying attention to which channel produces an output first, channel A or channel B gives two rotation directions.

Incremental Encoder Output

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One main drawback of incremental encoders when used as a position sensor is that they need external counters to determine the absolute angle of the shaft within a certain rotation.If the power is cut instantly or the encoder misses a pulse due to noise or dirty disk, the resulting angular information will produce an error.One way to overcome this disadvantage is to use absolute location encoders.

Absolute Location Encoder

Absolute Location Encoders aremore complex than square encoders.They provide a unique exit code for each return location that represents both the location and direction.Its encoded disks consist of a large number of concentric "traces" of light and dark partitions.Each track is independent of its own photo detector to read a unique encoded position value for each angle of motion at the same time.The number of shards on the disk corresponds to the encoder's binary "bit" resolution, so a 12-bit absolute encoder has 12 shards, and the same encoded value appears only once per revolution.

4-bit Binary Coded Disk

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One of the main advantages of the absolute encoder is its persistent memory, which maintains the encoder's exact position without the need to return to the "main" position if the power is cut off.Most rotary encoders are defined as "single-turn" devices, but there are absolute multi-turn devices that receive feedback over several cycles by adding extra code disks.

Typical application of absolute location encoders is found on computer hard drives and CD/DVD drives, the absolute location of drives, the monitoring of read/write heads, or printers/plotters to accurately position printheads on paper.

In this tutorial about Location Sensors, we looked at several examples of sensors that can be used to measure the position or presence of objects.In the next tutorial, we will look at sensors used to measure temperature, such as thermostats and thermocupators, and therefore are often known as Temperature Sensors.